A receiver of a satellite based positioning system obtains measured data from a plurality of orbiting satellites by observing the transmission delay between the emission of the radio signals from the satellites and their reception at the mobile receiver or, more particularly, an antenna of the mobile receiver. The position of the device which is to be determined is commonly referred to as a `mark`. Each receiver is equipped to receive and process the radio frequency signals. For a receiver to be able to estimate the position of the mark, certain requirements need to be met. Although the particular requirements depend on the particular type of positioning system, both knowledge of the orbits of several satellites and radio frequency signals from the orbiting satellites is required for all types of positioning system. The other requirements which may need to be met depend on the type of positioning system.
Knowledge of the orbits of each of several satellites is necessary if the method used to estimate the location of the mark is based on triangulation using estimates of the distance from each satellite to the mark. Knowledge of the orbit of each satellite is required so that the position of each one can be determined at the time of transmission of each of the radio frequency signals used for distance measurement. This is normally provided in mathematical form, valid within certain accuracy limits for a specific period of time.
Dissemination of the orbital data is usually in the form of a model and by way of data transmission from the satellite itself. Other techniques are known such as the publication of precise orbit data some time later, or transmission over a terrestrial data link. In any case, a format for the data is usually sought which provides a balance between complexity, accuracy and the duration of the applicability or validity of the orbital model.
The parameters used in such models are ephemeral. For ideal satellite bodies in orbit above a single, perfectly spherical, uniform host (planet), the orbits described would be exactly elliptical. These orbits would, in principle, persist for long periods of time. In practice, however, the satellite is subject to influences other than the obvious gravitational and centripetal forces. These include but are not limited to: the effects of other (celestial) bodies, such as the Sun, the Moon and other planets; the solar wind, which is the result of a stream of particles continuously emitted from the Sun; gravitational anomalies caused by the non-uniformity of the Earth; and the effects of a non-spheroidal Earth-the Earth is approximately an oblate spheroid.
Consequently, it is practical to provide models, which are each valid for limited periods of time, covering the orbit of a satellite in a series of segments. Such time variable parameters for an orbit description have become known as ephemeris models. The balance of complexity for such models is carefully judged and depends on many application sensitive factors including the work load necessary for maintenance of the ephemeris models. These balances are frequently optimised for the most efficient data transmission from the satellites, but may not be optimised for other applications or data links.
The ephemeris data for a GPS satellite is contained in a data message of 1500 bits length, transmitted over a period of 30 seconds, of which approximately 900 are involved in the description of the ephemeris model. These 900 bits are referred as ephemeris data.
The ephemeris data is valid for typically one or two hours, depending on the factors influencing the orbit of the satellite.
It is proposed in U.S. Pat. No. 5365450 to include in a positioning system using radio-frequency signal transmitting, orbiting satellites, a terrestrial station which receives ephemeris data from all "in-view" satellites and transmits the ephemeris data to a mobile station on a cellular telephone channel. The data is transmitted over the cellular telephone channel much more quickly than it is transmitted by the satellites, allowing the mobile station to more rapidly determine its position than previously.
It is an object of the present invention to provide an improved satellite orbital model transmission format for elements of a positioning system in which the data link is a terrestrial data link.